One aim of this project is to carefully monitor children who have had a prophylactic thyroidectomy on the basis of a positive RET protooncogene mutation, and to assist them for the development of advanced medullary thyroid carcinoma and other tumors. Another specific aim of the project is to develop a transgenic mouse having the mutated RET protooncogene, which should provide us an animal model for the MEN 2a and 2b syndromes.